This chapter is an examination of Williams’s account of substance or concrete objects. On his view, a concrete object is a mereological sum of tropes concurring in the same spatiotemporal region. It is argued that Williams’s account of concrete objects involves refined commitments to mereology. A concurrent sum of tropes is maximally concrete and trope-parts of objects occupy the exact region of the object such that a concurrent sum embraces its abstract parts. Although the notion of embracing is not transitive, it is argued that Williams is not committed to multiple modes of composition. He is able to maintain the view that there is only one mode of composition, which is governed by the principles of classical extensional mereology. The topic of essentialism about concrete objects is also examined. It is argued that he is an essentialist with respect to the universals expressed by tropes in a trope-bundle, but that he is an anti-essentialist with respect to the trope-parts of the object.